The useful life of a transformer largely depends on the drying process, both of the dielectric liquid wherein the windings are immersed and of its solid insulating materials, which, furthermore, must not have been damaged by the heating to which they have been subjected to be dried. In the same way, no air bubble can be permitted to subsist, or to be trapped during the filling-impregnation-polymerization process.
For immersed transformers, a two stage process is performed: firstly, a drying treatment in an oven, air or kerosen heated, whose duration depends on the importance of the transformer, but always exceeds several hours, and can reach several days.
With the increase of the production all over the world, the vacuum drying devices have been left, because they were too expensive, compared with the hot air drying ones. Only the oil filling stage is performed under vacuum, in small autoclaves that just contain a few transformers. However, the quality and swiftness of the vacuum drying treatment are admitted and missed. There are some low frequency heating techniques, performed under vacuum, but, as with the previous systems, their cost has prevented them from spreading.
For the power transformers that have tanks able to withstand vacuum, the two stages are directly performed in the shut or welded tank. The drying is performed through a succession of heatings followed by settings under vacuum. This process lasts several days. The oil filling under vacuum follows the drying stage.
Numerous small repairers or manufacturers do not possess vacuum filling devices because of their cost, which is very high regarding their capacity. None of the methods or devices commonly used can really be automatically driven or controlled. The manufacturers still take into account a high safety coefficient, empirically determined, that brings about production costs that are higher than one could except, especially in the developing countries, where needs are more important.